The present invention concerns a process for the manufacture of filled paper from a furnish containing mechanical pulp. In particular the invention includes processes for making highly filled mechanical paper grades, such as super calendared paper (SC-paper) or coated rotogravure (e.g. LWC).
It is well known to manufacture paper by a process that comprises flocculating a cellulosic thin stock by the addition of polymeric retention aid and then draining the flocculated suspension through a moving screen (often referred to as a machine wire) and then a forming a wet sheet, which is then dried. Some polymers tend to generate rather coarse flocs and although retention and drainage may be good unfortunately the formation and the rate of drying the resulting sheet can be impaired. It is often difficult to obtain the optimum balance between retention, drainage, drying and formation by adding a single polymeric retention aid and it is therefore common practise to add two separate materials in sequence or in some cases simultaneously.
Filled mechanical grade paper such as SC paper or coated rotogravure paper is often made using a soluble dual polymer retention system. This employs the use of two water-soluble polymers that are blended together as aqueous solutions before their addition to the thin stock. In general one of the polymers would have a higher molecular weight than the other. Both polymers would usually be linear and as water-soluble as reasonably possible. Usually the low molecular weight polymeric component would have a high cationic charge density, such as polyamine, polyethyleneimine or polyDADMAC (polymers of diallyl dimethyl ammonium chloride) coagulants. In contrast to the lower molecular weight polymers, the higher molecular weight polymeric component tends to have a relatively low cationic charge density. Typically such higher molecular weight polymers can be cationic polymers based on acrylamide or for instance polyvinyl amines. The blend of cationic polymers is commonly referred to as a cat/cat retention system.
In the general field of manufacturing paper and paperboard it is known to use other retention systems. Microparticulate retention systems employing siliceous material had been found to be very effective in improving retention and drainage. EP-A-235,893 describes a process in which a substantially linear cationic polymer is applied to the paper making stock prior to a shear stage in order to bring about flocculation, passing the flocculated stock through at least one shear stage and then reflocculating by introducing bentonite. In addition to wholly linear cationic polymers slightly cross-linked, for example branched polymers as described in EP-A-202780 may also be used. This process has been successfully commercialised by Ciba Specialty Chemicals under the trademark Hydrocol since it provides enhanced retention, drainage and formation.
Examples of other micro particulate systems used in papermaking industry are described in EP-A-0041056 and U.S. Pat. No. 4,385,961 for colloidal silica and in WO-A-9405596 and WO-A-9523021 with regard to silica based sols used in combination with cationic acrylamide polymers. U.S. Pat. No. 6,358,364, U.S. Pat. No. 6,361,652 and U.S. Pat. No. 6,361,653 each describe the use of borosilicates in conjunction with high molecular weight flocculants and/or starch in this sense.
In addition to inorganic insoluble microparticulate material organic polymeric microparticulate material is also known for papermaking processes.
U.S. Pat. No. 5,167,766 and U.S. Pat. No. 5,274,055 discuss papermaking processes with improved drainage and retention by using ionic, organic microparticles or microbeads having an average diameter of less than 750 nm if cross-linked and less than 60 nm if not cross-linked. The microparticles or microbeads are used in combination with high molecular weight ionic organic polymer and/or polysaccharide. The process may occasionally include alum.
US 2003 0192664 discloses a method for making paper by using vinyl amine polymers with ionic, organic, cross-linked polymeric microbeads. Optimisation of molecular weight, structure and the charge provide systems with improved drainage rate. The addition of different coagulants, such as polyethylene imine, alum or polyamine is said to further increase the drainage rate of these systems employing polymeric microbeads.
WO-A-9829604 describes a process of making paper by addition of a cationic polymeric retention aid to a cellulosic suspension to form flocs, mechanically degrading the flocs and then reflocculating the suspension by adding a solution of a water-soluble anionic polymer as second polymeric retention aid. The anionic polymeric retention aid is a branched polymer having a rheological oscillation of tan delta at 0.005 Hz of above 0.7 and/or having a deionised SLV viscosity number at least three times the salted SLV viscosity number of the corresponding polymer made in the absence of branching agent. The process provides significant improvements in retention, drainage and formation by comparison to the earlier prior art processes. It is emphasised on page 8 that the amount of branching agent should not be too high as the desired improvements in both dewatering and retention values will not be achieved.
U.S. Pat. No. 6,616,806 reveals a three component process of making paper by adding a substantially water-soluble polymer selected from a polysaccharide or a synthetic polymer of intrinsic viscosity at least 4 dl/g and then reflocculating by a subsequent addition of a reflocculating system. The reflocculating system comprises siliceous material and a substantially water-soluble polymer. The water-soluble polymer added before the reflocculating system is a water-soluble branched polymer that has an intrinsic viscosity above 4 dl/g and exhibits a rheological oscillation value of tan delta at 0.005 Hz of above 0.7. Drainage is increased without any significant impairment of formation in comparison to other known prior art processes.
U.S. Pat. No. 6,395,134 describes a process of making paper using a three component system in which cellulosic suspension is flocculated using a water-soluble cationic polymer, a siliceous material and an anionic branched water-soluble polymer formed from ethylenically unsaturated monomers having an intrinsic viscosity above 4 dl/g and exhibiting a rheological oscillation value of tan delta at 0.005 Hz of above 0.7. The process provides faster drainage and better formation than branched anionic polymer in the absence of colloidal silica. U.S. Pat. No. 6,391,156 describes an analogous process in which specifically bentonite is used as a siliceous material. This process also provides faster drainage and better formation than processes in which cationic polymer and branched anionic polymer are used in the absence of bentonite.
U.S. Pat. No. 6,451,902 discloses a process for making paper by applying a water-soluble synthetic cationic polymer to a cellulosic suspension specifically in the thin stock stream in order to flocculate it followed by mechanical degradation. After the centriscreen a water-soluble anionic polymer and a siliceous material are added in order to reflocculate the cellulosic suspension. Suitably the water-soluble anionic polymer can be a linear polymer. The process significantly increases drainage rate a comparison to cationic polymer and bentonite in the absence of the anionic polymer.
The prior art processes provide improvements in retention and drainage and often seek to improve the balance of retention, drainage and formation.
Nevertheless retention and drainage are increased simultaneously. None of the aforementioned prior art contemplates processes in which retention, in particular ash retention, is increased but drainage is maintained or reduced. Traditional papermaking processes have always placed emphasis on increasing retention and drainage in order to achieve higher productivity on paper machines as well as improving formation at the same time.
However, the introduction of paper machines that have extremely fast draining twin wire forming sections, frequently called Gapformers, have dramatically improved sheet building and paper stock drainage by mechanical means. Gapformer type paper machines are nowadays frequently used for the production of rotogravure printing papers, such as super calendared paper (SC) or light weight coated (LWC) papers. Gapformers drain the paper suspension fast enough so that especially for the lower basis weights between 34 and 60 g/m2 further enhanced drainage rates are not required. In some cases the Gapformers provide a high level of initial drainage. If this initial drainage becomes too high this can be adverse to functioning of the essential downstream shear and drainage elements in the Gapformers. This is because a minimum concentration of fibre suspension is required to apply the drainage pulses with high shear forces to optimise formation and z-directional sheet building.
A description of a Gapformer paper machine can be found in “Duoformer CFD—a new development in the field of sheet forming systems” from Schmidt-Rohr, V.; Kohl, B. J. M. Voith GmbH, Heidenheim, Germany Wochenblatt für Papierfabrikation (1992), 120 (11-12), 455-8, 460. In this document it is stated that the initial drainage with constant pressure at the forming roll results in high retention. The subsequent drainage by pressure pulses of opposing bars in the D-section enhances formation. Therefore, with the Duoformer CFD significantly improved formation can be achieved with improved retention. In the German addition of “Together—Magazin für Papiertechnik” (Issue 6 (1998), Böck, K.-J.; Moser, J.; published by Voith Sulzer Papiertechnik GmbH & Co. KG, editor Dr. Wolfgang Möhle, Corporate Marketing, Voith Sulzer Papiertechnik GmbH) it is stated under superscription “D-section (foil or blade section)” that the sheet building in z-direction can be controlled effectively. However it is important that fibre is still in the form of a suspension in order to allow mobility of the fibres. It is further explained that due to the D-section, very good results are achieved. It is stated that by increasing the dewatering in the D-section, formation improves dramatically.
In the trade publication from the J. M. Voith GmbH (“Triple Star”—The state of the art and most efficient production line in the world for woodfree coated papers; Kotitsche, G., Merzeder, K.-D. and Tiefengruber, M. from Sappi Gratkorn GmbH; Voith trade publication p 316e, 6.98 4000, page 7, column 2, paragraph 3, FIG. 8) it is stated that “the flow rate to be drained in the foil section of the former must be as high as possible. In this way uniform and soft formation is achieved.”
The aforementioned principles are still valid also for the newest generation of Gapformers. In Voith trade publication p 3276 e 4000 2002-06 “Duoformer TQv” is stated that the curved suction box and loaded forming blades, also known as the D-section, are prerequisites for excellent formation. The box has two chambers for dewatering and controlling sheet structure in z-direction. It is further stated that “in combination with the furnish quality, two main parameters influencing formation were found, regardless of the grade: the use of the forming blades and the white water flow rate in the forming shoe. A high forming shoe flow rate improves formation in any situation, whether the forming blades are loaded or not. This is caused by the effect that the forming blades work best when the suspension is liquid enough to allow fibre movements.
Another example again stresses the importance of controlled initial drainage in gapformers, e.g. designed and engineered in accordance with WO-2004018768. Metso trade publication EN—03 (December 2004) states that the BelBaie V gapformer delivers “better formation thanks to gentle initial dewatering and loadable blades (page 1). Further information can be found in “Bel Baie V upgrade” (Swietlik, Frank; Irwin, Jeff; Jaakkola, Jyrki. Metso Paper USA, Norcross, Ga., USA. Preprint—Annual Meeting, Pulp and Paper Technical Association of Canada, 90th, Montreal, QC, Canada, Jan. 27-29, 2004 (2004), Book A A109-A112. Publisher: Pulp and Paper Technical Association of Canada, Montreal, Que).
The comparable situation also applies to hybrid formers, in which the sheet is formed on a conventional Fourdrinier table, and then a top wire with dewatering elements is applied in the same manner. A general description of this hybrid former can be found in “Sheet forming with Duoformer D and pressing with shoe presses of the Flexonip type for manufacturing of linerboard and testliner, corrugating medium and folding boxboard” (Grossmann, U.; J. M. Voith GmbH, Heidenheim, Germany. Wochenblatt für Papierfabrikation (1993), 121(19), 775-6, 778, 780-2.). The control of drainage is crucial for sheet building and final product quality.
It is clear that simply increasing drainage in many cases will not provide the solution to obtaining optimised paper quality. On the contrary it would be desirable to provide controlled drainage.
Although increased dewatering in the blade section can be achieved by increasing the fan pump speed which will carry more water through into the forming zone, adjusting drainage elements, reducing headbox solids and/or reducing the initial drainage on the forming roll it would nonetheless be desirable to provide chemical means that optimise paper quality. In particular it would be desirable to provide a chemical retention system that would allow a decreased drainage rate but enhances retention. In particular it would be desirable to optimise sheet building combined with adequate ash retention in order to reach the desired filler level in addition to optimising floc size distribution. It would especially the desirable to achieve this in addition to producing finer/smaller aggregates for improved formation. Furthermore, it would be desirable to provide a process that provides increased ash retention, and preferably formation, and maintaining or preferably reducing drainage for filled mechanical grade papers.